The present invention relates to a semiconductor device, and more particularly to an overlay vernier key and a method for fabricating the same.
In manufacturing a semiconductor device having a laminate structure, an overlay is an index indicating an alignment state between a layer formed in a previous process and a layer formed in a current process. The overlay is very important when developing a highly integrated semiconductor device. In order to detect and correct the alignment state between the layer formed in the previous process and the layer formed in the current process, the overlay between the layers are measured by forming an overlay vernier key in a scribe lane region. The overlay vernier key is a pattern which is formed for aligning an exposure mask at an accurate position when forming a predetermined pattern on a semiconductor substrate. The overlay vernier key is formed simultaneously with a pattern formed in a device forming region.
FIG. 1A is a schematic view showing a conventional structure of an overlay vernier key. FIG. 1B illustrates an overlay measuring signal detected when measuring a conventional overlay vernier key.
Referring to FIG. 1A, an overlay vernier key 10 has a box-shaped pattern. The overlay vernier key 10 has a larger size and pitch than a pattern formed in a cell region. In a highly integrated device, a margin is decreased and the overlay vernier key 10 does not have a sufficient line width. Thus, it is difficult to measure the overlay with optical overlay measuring equipment and to form a micro pattern in the cell region.
A Spacer Double Exposure Technique (SDET) is a method for forming the micro pattern under 60 nm on a wafer “w” (see FIG. 1B). The SDET method includes the steps of forming a dummy pattern on a substrate, forming an etch mask having a spacer shape at a side surface of the dummy pattern, removing the dummy pattern, and forming the micro pattern and overlay vernier key using the etch mask.
As shown in FIG. 1B, when forming the overlay vernier key 20 using the SDET method, the spacer-shaped etch mask is formed at a sidewall of the dummy pattern (not shown) in a scribe lane region, which is larger than the cell region. The size of the etch mask in the scribe lane region is same as that of the etch mask in the cell region. The overlay vernier key 20 formed by using the etch mask has a line width which is much larger than a proper line width. As seen in FIG. 1B, because the overlay measuring signal is too weak to be detected by the optical overlay measuring equipment, the optical overlay measuring equipment cannot perceive a contrast difference. If the overlay vernier key 20 is formed to have a thickness large enough to be measured by the optical overlay measuring equipment, there may be a problem caused by a difference between the size of the overlay vernier key 20 and the size of the pattern in the cell region. Therefore, the overlay vernier key is required to have a structure adequately sized to be measured by the optical overlay measuring equipment.